The present application generally relates to eye and head tracking. In particular, though not exclusively, the present application relates to non-contact eye and head tracking.
Eye movements of human beings are mediated by different voluntary and involuntary mechanisms. Different parts of the nervous system are responsible for starting, maintaining and terminating eye movements. Fixation is the maintaining of the visual gaze on a single location. Other parts of the nervous system are responsible for maintaining eye fixation on an object regardless of the head position in space.
The vestibulo-ocular reflex (VOR) or the oculo-cephalic reflex is a reflex eye movement that stabilizes images on the retina during head movement by producing an eye movement in the direction opposite to the direction of head movement, thus preserving the image on the center of the visual field. This reflex compensates for horizontal, vertical and rotational head movements. For example, when the head moves to the right, the eyes proportionately move to the left while if the individual's head tilts to towards the shoulder, both eyes rotate along their antero-posterior axis in a movement called torsion to compensate for the head tilt.
Tracking of the eye movements has various applications such as detecting of abnormal eye movements due to neurological disorder. Tracking of the eye movements can also be used to move a cursor on a computer screen so that the eyes of the user can control such a device. Eye trackers measure rotations of the eye in one of several ways, but principally they fall into three categories: (i) measurement of the movement of an object (normally, a special contact lens) attached to the eye; (ii) optical tracking without direct contact to the eye; and (iii) measurement of electric potentials using electrodes placed around the eyes.
The first category generally uses an attachment to the eye, such as a special contact lens with an embedded mirror or magnetic field sensor, and the movement of the attachment is measured. Measurements with tight fitting contact lenses have provided extremely sensitive recordings of eye movement. It allows the measurement of horizontal, vertical and torsional eye movements.
The second broad category uses non-contact, optical method for measuring eye motion. Light, typically infrared, is reflected from the eye and sensed by a video camera. The information is then analyzed to extract eye rotation from changes in reflections. Video-based eye trackers typically use the corneal reflection (the first Purkinje image) and the center of the pupil as features to track over time. Optical methods, particularly those based on video recording, are widely used for gaze tracking and are favored for being non-invasive. Such trackers typically require relatively high resolution cameras capturing at a high frame rate with image processing and pattern recognizing device to track the reflected light or known ocular structures such as the iris or the pupil. They lack the accuracy of the contact method and cannot detect torsional eye movement along its antero-posterior access.
The third broad category involves the use of electrodes placed on the surface of the eye to measure electric potentials. A limitation of this method is the inaccuracy of measurements for small and large eye movements. Background electric noise from the brain and facial muscles that is equivalent to approximately 1 degree of eye movement appears on the signal. Therefore, eye movements that are less than 1 or 2 degrees are difficult to record using this method. Also, eye movements that are greater than approximately 30 degrees do not produce amplitudes that are strictly proportional to eye position
As discussed above, while the use of current non-contact eye tracking devices are less invasive and presumably more comfortable to the patient during examination, their use generally results in a level of reduced efficacy and accuracy. While there are attempts to meet this need in the prior art, such set forth in PCT application WO2013110846 to Bergman et. al., these attempts fall short. For example, Bergman discloses the use of a capacitative sensor for non-contact eye tracking; however, capacitative sensors have limitations in that they are very sensitive to environmental conditions such as humidity and temperature. Since the ocular surface is continuously evaporating tears, the accuracy of the capacitative sensor is impaired. Further, capacitative sensors are very sensitive to the distance from the eye, so even minor changes in position of the frames used in Bergman can affect accuracy. Additionally, movement of the eyelid in front of the eye (blinking) can be misinterpreted by capacitative sensors as eye movement.
Optical flow or optic flow is the pattern of apparent motion of objects, surfaces, and edges in a visual scene caused by the relative motion between an observer (an eye or a camera) and the scene. The concept of optical flow was introduced by the American psychologist James J. Gibson in the 1940s to describe the visual stimulus provided to animals moving through the world. Recently the term optical flow has been co-opted by roboticists to incorporate related techniques from image processing and control of navigation, such as motion detection, object segmentation, time-to-contact information, focus of expansion calculations, luminance, motion compensated encoding, and stereo disparity measurement.
Optical flow was used by robotics researchers in many areas such as: object detection and tracking, image dominant plane extraction, robot navigation, movement detection and visual odometry. Optical flow information has been recognized as being useful for controlling micro air vehicles and robots.
An optical flow sensor is a vision sensor capable of measuring optical flow or visual motion and outputting a measurement based on optical flow. Various configurations of optical flow sensors exist. One configuration is an image sensor chip connected to a processor programmed to run an optical flow algorithm. Another configuration uses a vision chip, which is an integrated circuit having both the image sensor and the processor on the same die, allowing for a compact implementation. An example of this is a generic optical mouse sensor used in an optical mouse. In some cases the processing circuitry may be implemented using analog or mixed-signal circuits to enable fast optical flow computation using minimal current consumption.
One area of contemporary research is the use of neuromorphic engineering techniques to implement circuits that respond to optical flow, and thus may be appropriate for use in an optical flow sensor. Such circuits may draw inspiration from biological neural circuitry that similarly responds to optical flow.
Optical flow sensors are used extensively in computer optical mice, as the main sensing component for measuring the motion of the mouse across a surface.
Optical flow sensors are also being used in robotics applications, primarily where there is a need to measure visual motion or relative motion between the robot and other objects in the vicinity of the robot. The use of optical flow sensors in unmanned aerial vehicles (UAVs), for stability and obstacle avoidance, is also an area of current research.
Therefore, there is a need in the art for non-contact eye tracking device providing a more simplified operation and more precise motion tracking. Additionally, there is a need for a non-contact device that can detect torsional eye movement along the antero-posterior access of the eye without contacting the eye.